1. Field of the Invention
This invention relates generally to honeycombed structures formed of ceramic materials. More specifically, this invention relates to the forming of ceramic materials into thin wall honeycombed structures by extrusion and the coating of the channels in the structure with a fluid.
2. Description of the Prior Art
The term honeycombed structures is used generally to describe a thin-walled body having a series of regularly or irregularly shaped parallel channels that extend continuously over the length of the body and are separated by wall elements that give the body its structure. The cross-section of each channel may vary from channel to channel but usually will have a regular geometric shape. These honeycombed structures find use in regenerators, heat exchange equipment, filters, and as catalyst carriers. The use of such carriers is also well known in the treatment of automotive exhaust gases where the carriers are typically treated with a wash coat of catalytic material.
Ceramic honeycombs have been formed by extrusion methods with fairly good success. The extrusion method uses a hydraulic ram to push the extrudable material into a series of feed passages which communicate with a discharge area. The discharge area has a series of projections that displace the extrudable material from the sections that will eventually correspond to the channels and define a series of gaps which shape the extrudable material into the walls of the honeycombed structure. It has become common practice to extrude honeycombs having channel densities of from 80 to 450 channels per square inch upon extrusion, and 100 to 600 channels per square inch after shrinkage of the extrudable material during curing. Typically, the wall thicknesses between the channels of the honeycombed structure will vary between 0.002 and 0.050 inches.
When the honeycombed structures are used as catalyst carriers, a portion of the catalytic material is usually coated onto the sidewalls of the channels. A fluid containing the desired catalytic material contacts the channel walls to deposit the material on the surface of the channel. In the case of automotive exhaust gas purification, the fluid is usually a liquid referred to as a wash coat that contains an alumina component and one or more additional metal oxide components as promoters. Generally preferred metal oxide components present in the wash coat are ceria, titania, zirconia, lanthanum oxide, copper oxide, and nickel oxide. Prior art methods apply the wash coat to formed honeycombed structures by spraying the wash coat into the channels under pressure or immersing the honeycomb into a wash coat bath. Regardless of the method employed, wash coating of the monolith requires at least one extra procedure in the manufacture of most catalytically active honeycombed structures.
In addition to requiring an extra step, it is often difficult to apply the wash coat in a manner that will uniformly cover the channel walls. Either the wash coat material does not completely coat the channel walls, or excessively thick deposits of wash coat materials may be found in corners of the polygonally shaped channels. Incomplete wash coat coverage fails to make complete utilization of the honeycombed structure surface while excessive deposits waste valuable catalytic materials and reduce the overall surface area of the channels. These problems stem, at least in part, from the high aspect ratio of the fine channels and the resulting difficulty in getting wash coat down the entire channel length.